In the treatment of hypertension to reduce the risks of complications such as coronary heart diseases and cardiovascular diseases, e.g., stroke, heart failure and myocardial infarction, it is more important to maintain the blood pressure within a normal range on a consistent basis than to simply lower the blood pressure level itself. Accordingly, antihypertensive agents are required to be effective for long-term treatment of hypertension. Further, advanced therapy using a combination of two or more drugs having different pharmacological actions makes it possible to improve preventive or therapeutic effects, while lowering side effects arising from the long term administration of a single drug.
Notable antihypertensive drugs include diuretics, sympatholytic agents and vasodilators. Vasodilators are most widely prescribed antihypertensive drugs, and they are divided into several groups according to their pharmacological actions which include ACE (angiotensin converting enzyme) inhibitors, angiotensin II receptor antagonists, and calcium channel blockers.
Amlodipine is the generic name for 3-ethyl-5-methyl-2-(2-aminoethoxy-methyl)-4-(2-chlorophenyl)-6-methyl-1,4-dihydro-3,5-pyridine dicarboxylate. Amlodipine besylate is currently marketed as Novasc (trade mark). Amlodipine is a long-acting calcium channel blocker which is useful in treating cardiovascular disorders such as angina, hypertension and congestive heart failure.
Losartan is the generic name for 2-butyl-4-chloro-1-[[2′-(1H-tetrazol-5-yl)[1,1′-biphenyl]-4-yl]methyl]-1H-imidazol-5-methanol, which has been disclosed in U.S. Pat. Nos. 5,608,075; 5,138,069; and 5,153,197. Losartan potassium is commercially available as Cozaar (trade mark). Losartan blocks the interaction of angiotensin II and its receptor, and is mainly used for treating hypertension, heart failure, ischemic peripheral circulatory disorder, myocardial ischemia (angina pectoris), diabetic neuropathy and glaucoma, and also for preventing the progression of post-myocardial infarction heart failure.
The present inventors have found that a combined formulation which comprises amlodipine and losartan having different pharmacological activities is useful for preventing or treating cardiovascular disorders, and have conducted intensive studies on such a combined formulation. However, the development of a stable amlodipine-losartan combined formulation which can be reproducibly and easily prepared has been hard to achieve mainly because of difficulties in handling the two drugs.
Amlodipine is generally used in the formulation in the form of an acid-addition salt with a pharmaceutically acceptable acid which is more stable and exhibits a higher water-solubility than a free base form of amlodipine.
It is reported that amlodipine malate, one of such acid-addition salts of amlodipine, tends to gradually decompose with time after formulation into amlodipine aspartate of formula (I) or amlo-pyridine of formula (II), to lower the efficacy of a pharmaceutical composition comprising same (U.S. Pat. No. 6,919,087). Amlodipine besylate which is commercially available and disclosed in EP 244944 (corresponding U.S. Pat. No. 4,879,303) has been recently placed in general use, but it also suffers the above-mentioned problems such as decomposition and poor storage stability.

The present inventors have developed camsylate salt of amlodipine which exhibits improved properties in terms of solubility and stability than amlodipine besylate, and the camsylate salt is currently marketed as Amodipine (trade mark). It has been found, however, that when formulated by simple mixing with losartan, amlodipine camsylate exhibits very poor storage stability presumably due to undesired chemical interactions among amlodipine, losartan and excipients.
It is well known that when heated under an acidic condition, losartan potassium also decomposes to form products referred to as degradate E or F (see [Z. Zhao et al., J. Pharm. Biomed. Anal, 20: 129-136, 1999]). Further, in case that losartan is formulated in the form of a combined formulation by simple mixing with an acid-addition salt of amlodipine, the acidic component of the amlodipine salt destabilizes losartan.